Problem: In chemistry class, the girl to boy ratio is $8$ to $4$. If there are $32$ girls, how many boys are there?
Answer: A ratio of $8$ girls to $4$ boys means that there is a group of $8$ girls for every group of $4$ boys. If there are $32$ girls, then there are $4$ groups of $8$ girls each. Because we know the ratio is $8:4$ , there must be $4$ groups of $4$ boys each. There is a total of $16$ boys in chemistry class.